A web service may evolve autonomously, making peer web services in the same service composition uncertain as to whether the evolved behaviors may still be compatible to its originally collaborative agreement. Although peer services may wish to conduct regression testing to verify the original collaboration, the source code of the former service can be inaccessible to them. Traditional code-based regression testing strategies are inapplicable. The rich interface specifications of a web service, however, provide peer services with a means to formulate black-box testing strategies. In this paper, we formulate new test case prioritization strategies using tags embedded in XML messages to reorder regression test cases, and reveal how the test cases use the interface specifications of services. We evaluate experimentally their effectiveness on revealing regression faults in modified WS-BPEL programs. The results show that the new techniques can have a high probability of outperforming random ordering.